1. Field of the Invention
The present invention relates to a method of fabricating a steel strip which will display high strength and formability properties, the initial steel used in forming the steel strip having a low carbon content and including very low amounts of alloying compounds.
2. The Prior Art
In order to produce steels for applications where high strength as well as good formability are required, the so-called dual-phase steels have been developed which are characterized by having a micro-structure of fine-grained, polygonal ferrite with grains of martensite dispersed therein. The strength of such steels is determined mainly by the volume fraction of martensite, whereas the ductility is determined by the volume fraction of ferrite. Thus, as the amount of martensite increases from 5 to 25%, the tensile strength of the steel will vary between approximately 400 and 1,400 MPa, and the elongation thereof will vary between 40 and about 10%.
To develop this internal structure in the steel in a steel strip, an annealing treatment can be utilized which involves heating the steel strip to a temperature above the transformation point A.sub.1 in the iron-carbon diagram (usually to about 750.degree. C.), followed by a quick cooling from this temperature (such a quick cooling being achieved, for example, by spraying the steel strip with water or blowing a cooling gas against it). However, such an annealing treatment involves considerable costs, i.e., since such treatment requires the use of much energy and presupposes the use of technically complicated equipment.
One way to avoid these extra costs is to use as the initial steel a steel having suitable alloying compounds therein such that with a suitably elaborated cooling of the hot-rolled steel strip, the desired internal structure will be obtained directly. Such a method is described in U.S. Pat. No. 4,072,543. The advantage of this method is that no heat treatment of the steel strip is needed after the rolling thereof; however, this technique is expensive since the initial steel must include fairly expensive alloying materials, such as molybdenum in amounts of up to 0.4%. In addition, a powerful cooling means must be located downstream of the hot-strip mill, which will be both expensive and troublesome since modern hot-strip mills operate with a high rolling velocity.
It is thus an object of the present invention to provide a method of fabricating a steel strip which will be composed of a dual-phase steel having a high strength and formability, but which will avoid the need to use expensive initial steels and/or expensive and troublesome processing steps required in prior art steel strip fabricating techniques.